The present application claims priority of Chinese patent application Ser. No. 200610011944.4, filed May 19, 2006, the content of which is hereby incorporated by reference in its entirety.
The existing cargo inspection system based on radiographic imaging generally causes a single energy radiation to interact with the object under inspection, and then detects the radiation having penetrated the object under inspection to obtain an image. Although such a system can reflect the change in shape and mass thickness of the object under inspection, it can't discriminate the material property of the object under inspection.
As the concern on global anti-terrorism grows, the inspection requirements for dangerous and prohibited articles are enhanced, whereby various detection means being proposed, in which a material discrimination method employing dual-energy X-ray imaging can achieve the discrimination between substances' effective atomic numbers, therefore is widely used in low energy range (<450 keV). As is well known, photoelectric absorption and Compton scatter effects predominate when X-rays within low energy range interact with the substance. Since the relationship between attenuation index corresponding to photoelectric absorption effect and atomic number is μphoto∝Z4, the dual-energy method is well capable of distinguishing the difference between various atomic numbers.
However, when X-rays within high energy range (>1 MeV) interact with the substance, electron pair generation and Compton scatter effects predominate, and the relationship between attenuation index corresponding to electron pair generation effect and atomic number is μpair∝Z, which makes the sensitivity of the dual-energy method low in distinguishing atomic numbers in high energy range. Thus the requirement for the system detection accuracy is very high. The U.S. Pat. No. 6,069,936 and the international application WO 0043760 both disclose that a single high-energy X-ray source is employed, and two X-ray beams having different energy spectra are obtained through the absorption of specific material. However, since a single high-energy X-ray generates only one original energy spectrum, the energy spectra of two radiation beams obtained through the absorption of specific material will become almost the same after the two ray beams having different energy spectra pass through the inspected object of large mass thickness and attenuate accordingly. At this time it is impossible to distinguish the effective atomic number of the substance. If two beams of high-energy X-rays having different original energy levels and spectra are generated respectively by using tow radiation sources in order to identify material, the system will be too complex and costly.
Consequently, it has been considering as impractical to implement large-sized object inspection and material discrimination by the dual-energy method in high energy range.